This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The assembly of neural circuits in the developing brain is thought to require both genetically determined and activity-dependent mechanisms. Many of the long-term changes mediated by neural activity are dependent on calcium-activated transcription of new gene products. Thus, calcium-activated transcription factors provide a mechanism for linking extrinsic signals with genetic programs of neural development. NeuroD2 is a basic helix-loop-helix transcription factor previously isolated in our lab, using a screen for novel calcium-regulated transcription factors involved in cortical development. NeuroD2 expression is developmentally regulated in the hippocampus, peaking between P7 and P14, coincident with the primary period of synaptogenesis in this structure. Here, we demonstrate a role for NeuroD2 mediated transcription in the normal development of mossy fiber (MF) to CA3 connectivity. In NeuroD2 null mice, which have grossly normal brain structure, calbindin staining of the MF afferents revealed a dramatic and preferential reduction in the distal portion of the main MF bundle. Although the fiber tract extends the full length of the CA3 region, it fails to elaborate the characteristic enlargement known as the [unreadable][unreadable][unreadable]end bulb[unreadable][unreadable][unreadable] at its distal tip. TIMM staining for pre-synaptic terminals in the MF pathway revealed a similar, if not greater, reduction in synapse formation in this pathway. Analysis of post-synaptic morphology of CA3 pyramidal neurons by Lucifer Yellow dye injection revealed a similar reduction in the characteristic multi-headed spines of the proximal apical dendrites receiving MF input. Interestingly, the size and density of the more distally located classical spines was not affected in the KO. This raises the possibility that while NeuroD2 mediated transcription is required for the development of MF connectivity, synapses from other pathways onto the same neuron may form in a NeuroD2 independent manner. These studies begin to define a novel transcriptional pathway regulating the development of connectivity in a major excitatory component of hippocampal circuitry.